With regard to an audio communication technique for transmitting data by means of sound waves propagating through a medium, such as air, a technique is known in which a data signal undergoes spectrum spreading and is emitted as a spread signal (see Patent Literature 1). The spread signal becomes unpleasant noise to human-beings. Thus, with the technique of Patent Literature 1, the spread signal is mixed with an audio signal or the like to control such that the signal level of the spread signal is equal to or smaller than a masking threshold value.
A code transmission technique in which audio is used as a transmission medium is also described in Patent Literatures 2 and 3. Patent Literature 2 describes a method in which a carrier wave in an audible sound band is modulated with a baseband signal and the modulated signal is transmitted to be not easily heard as a masker sound. Patent Literature 3 describes a method in which amplitude modulation is used to embed an electronic watermark in an audio signal.